There have been numerous prior proposals relating to the underwater surface of inflatable boats, and it has also been suggested that for rigid-hull boats multi-keel underwater surfaces should be provided. Thus in British Pat. No. 1,273,692 there is disclosed a boat hull provided in a forward section with a twin keel and in a rearward section with a single keel. The forward twin keels may each be lowerable or raisable to improve riding characteristics of the hull.
In British Pat. No. 1,319,072 there is disclosed an inflatable boat in which the shape of the hull is a function of the hull construction itself of a number of inflatable tubes which cooperate to provide a generally U-shaped or V-shaped cross-section for the hull.
In British Pat. No. 1,362,677 there is disclosed an inflatable boat having inflatable side walls serving as floatation members, a flexible underwater skin, a rigid deck and between said skin and deck an inflatable former member along the longitudinal centre-line of the boat shaping the flexible skin to form a central keel.
However in none of these prior art proposals has it been appreciated or suggested that by having a boat with a flexible underwater skin and inflatable former members to cause the undersea surface of the boat to have a multi-keel construction, automatic adaptation of the planing characteristics of the boat to the load carried in the boat could be achieved.